Laser micro-machining with state of the art solid state lasers typically involves the use of lasers with galvanometer scanners to position a focussed laser beam on the surface of a wafer or substrate to be machined. Typically, these lasers operate at repetition rates of 30 to 200 kHz and scanning occurs at velocities such that individual focussed laser spots overlap to some extent to form a shallow trench or scribe line. As shown in FIG. 1, multiple lines or passes 11 are used in this way to cut through or to dice, i.e. singulate, semiconductor devices 12. This process is described in, for example, EP 1328372.
Thus laser dicing is accomplished by scanning a laser beam across a substrate. Typically, with laser dicing, as shown in FIG. 2, successive adjacent pulses 21 are placed to have a certain overlap by scanning the laser beam at a particular scan speed to give a substantial overlap between the pulses. The scan is repeated in a number of passes 11, until the substrate is fully diced through. As shown in FIG. 3, this results in dicing with a relatively smooth edge 31. As shown in FIGS. 4 and 5, if the overlap of pulses 41 is reduced the die can appear to have a ‘scalloped’ edge 51.
Moreover, as shown in FIG. 6, when a laser pulse 61 ablates material 62 a plume of debris 63 is created. If the subsequent pulse 64 interacts with the debris plume 63, the succeeding laser pulse 64 is partially attenuated because the debris plume partially absorbs the energy of the succeeding laser pulse.